Many technologies have been developed and/or demonstrated for utilizing high-sulfur fuels in general and coals in particular. From a performance, emissions, and economics standpoint, fluidized bed combustion technology has emerged as a leading candidate for utilizing high sulfur fuels. Many fluidized bed combustion engines are available and are at various states of commercialization. Such systems can be classified in terms of operating pressure (atmospheric or pressurized) and fluidization mode (bubbling or circulating). All the fluidized bed designs possess attributes such as in-situ sulfur capture, no slagging or fouling of heat transfer surfaces, high heat transfer rates to heat exchange surfaces, near uniform temperature in combustion zone, and fuel flexibility. These features have made it possible for fluidized bed combustion technology to compete successfully for the large industrial boiler market (6.3-37.8 kg/s or 50,000-300,000 lb/hr steam). Large-scale (70 to 150 MW.sub.e) field demonstration projects are in progress to facilitate commercialization in the utility sector. The potential of fluidized bed combustion technology, and specifically, atmospheric fluidized bed combustion for small-scale (&lt;6.3 kg/s or 50,000 lb/hr steam equivalent) applications have, however, not been explored seriously until recently.
Atmospheric fluidized bed combustion technology appears to have a great potential for oil and gas replacement in small-scale installations of less than 6.3 kg/s (50,000 lb/hr) steam equivalent. These smaller units can meet the needs of process heat, hot water, steam, and space heating in the residential, commercial, and industrial sectors. Currently, oil and natural gas-fired equipment are being used almost exclusively for these applications. Due to the large difference between the prices of these fuels and coal, coal-fueled atmospheric fluidized bed combustion technology engineered for small-scale applications has the potential of becoming very competitive under economic conditions in which the price differential overcomes the initial capital cost of the coal-based system. A successful coal-fueled system cannot only be more economical, but can also reduce the nation's dependence on foreign oil and open up new markets for domestic coal and the coal-fueled fluid-bed technologies.
Market analysis indicates that a coal-based system that provides competitive levels of capital and operation and maintenance costs, performance, and reliability at the 0.126 to 1.26 kg/s (1,000 to 10,000 lb/hr) steam generation rate can displace as much as 2.64 EJ (2.5 quad Btu) of gas and oil within the residential, commercial, and light industrial sectors. In the industrial sector, systems from 1.26 to 6.3 kg/s (10,000 to 50,000 lb/hr) steam can displace another 1.16 EJ (1.1 quad Btu) of energy per year.
As pointed out earlier, the atmospheric fluidized bed combustion systems can be classified into bubbling-bed and circulating-bed systems. In a coal-fueled bubbling-bed system, it is critical to control the extent of fines (elutriable particles) in the coal and sorbent feed in order to limit particle carryover and its adverse effect on combustion and sulfur capture performance, emissions, and the size of solids collection equipment. Additionally, the higher Ca/S feed ratios typically required in bubbling fluidized combustion applications tend to increase sorbent and waste disposal costs, and turndown capability is rather limited. A circulating fluidized bed combustion system exhibits higher combustion efficiency and sorbent utilization, lower NO.sub.x emissions due to multiple air staging, and greater fuel flexibility and turndown as compared to a bubbling type system. However, the circulating type system requires a tall combustor to accommodate sufficient heat exchange surface. Such makes it both impractical and expensive to scale-down circulating fluidized bed combustors to sizes significantly smaller than 12.6 kg/s (100,000 lb/hr) steam equivalent.
Fluid bed systems in general tend to have large thermal inertia. Start-up for large fluid bed systems requires a considerable amount of time and also auxiliary subsystems to preheat the beds in a controlled manner. Both add to overall system cost and complexity. Concepts which provide a simple compact design for fast start-up with low-cost hardware and also have simple operational characteristics are a must for small-scale applications. Thermal inertia of fluid bed systems also affects load following to some extent and this has also been a serious shortcoming for scale-down to small end-use applications. System designs must provide fast response to load changes, particularly through auxiliary firing subsystems and methods of bed heating. Such designs should not require additional hardware and control systems if the system capital cost is to be maintained sufficiently low to compete favorably with existing oil and gas equipment. In addition, new designs capable of higher throughput for a given combustor size will contribute to a reduction in capital cost per kJ/hr (Btu/hr) of fuel fired. This must be achieved, however, without compromising the pollution control performance of equipment intended to meet stringent requirements in some of these end-use applications.
Simply scaling-down existing large atmospheric fluidized bed combustion systems to a size range suitable for small end-use sectors of interest will result in complex and expensive systems that will not be competitive with presently available oil- and gas-fired equipment. New innovative approaches are needed to reduce cost and enhance performance.
Such a new system should therefore possess a number of attributes, such as high combustion efficiency; high sulfur capture capacity; low N.sub.x emissions; and should be capable of rapid start-up with load-following capability. Also, such systems, as with most systems, should be of a simple design with inexpensive, easily managed controls to afford a reliable, safe system. Last, but not least, the system should be at least technologically and economically equivalent to oil- and gas-fired packaged systems.
The apparatus and process according to the present invention overcome the above-noted problems of the prior art and possess the attributes set forth above.